For example, a drum washing machine is provided with an electric motor which rotates a drum serving as a rotating tub rotatably mounted in a water tub. In the washing machine, the drum is rotated at low speeds in a positive direction or a reverse direction in each of wash and rinse steps, and the rotating tub is rotated at high speeds in one direction in an intermediate or final dehydration step. The drum is thus rotated at high speeds so that centrifugal dehydration is carried out.
The motor used in the above-described washing machine necessitates low-speed rotation and high torque in a washing operation including rinsing, whereas the motor necessitates high-speed rotation and low torque in a dehydrating operation including the intermediate dehydration and final dehydration, as shown in FIG. 9. In conventional motors, however, motor characteristics cannot be changed. Accordingly, a maximum efficiency point A of the motor is set between wash and dehydration as shown in FIG. 10. This results in a problem that the motor cannot efficiently be operated at each operation step.
In order that the above-described problem may be coped with, there is disclosed a technique of switching motor characteristics.
Although the motor characteristics can be switched so as to be suitable for the operation in each step in the aforementioned technique, the switch (the relay) is necessitated to switch between the Y-connection and the delta connection. The motor has a defect that the arrangement thereof is complicated.